When clipping occurs on a path in a telephone line and a VoIP network, harmonic distortion occurs and the audio quality of communications is degraded. For example, as illustrated in FIG. 1, there can occur clipping in any of the amplifiers (amp) 1601, 1602, 1603, etc. on the path. In this case, means for detecting the clipping by monitoring a communication signal during the conversation at any point as indicated by 1604 on the path may be used.
Published technology of the clipping detecting method is described in the following patent document 1. In the prior art, for example as illustrated in FIG. 2, input sound is converted into a digital audio signal by a ADC (A/D converter) 1701 and a digital amplifier 1702 for amplifying the digital audio signal is provided on the path at a transmitter, and the transmitted digital audio signal is converted into an analog audio signal by a DAC (D/A converter) 1703 at a receiver and the analog audio signal is amplified by an analog amplifier 1704, thereby outputting the sound from a speaker.
With the above-mentioned configuration, a clipping period detection unit 1705 compares the maximum value and the minimum value of the amplitude level of a signal with the maximum value and the minimum value of the line of audio distortion as illustrated in FIGS. 3A and 3B using the audio signal (input sound) before the amplification and the amount of amplification in the digital amplifier 1702 or the analog amplifier 1704 at a subsequent stage, and detects the period in which clipping is expected when the amplification is performed by the digital amplifier 1702 or the analog amplifier 1704. Then, on the basis of a detection result, a waveform correction unit 1706 illustrated in FIG. 2 makes a correction to suppress the amplification by the digital amplifier 1702 or the analog amplifier 1704 at the subsequent stage. As a result, an output analog audio signal is corrected from a first analog audio signal AS1 to a second analog audio signal AS2 as illustrated in FIG. 3A→FIG. 3B, thereby moderating the distortion by the clipping.
The related technology is described in, for example, the following patent document 1.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2001-83998
However, the published technology described in the patent document 1 may not detect the clipping unless the amount of amplification by the digital amplifier 1702, the analog amplifier 1704 etc. at the subsequent stage is known, and therefore it may not applicable in monitoring the sound quality on the path of a telephone line.